The present invention disclosed herein relates to a method of filtering a digital signal, and more particularly, to a method of filtering a digital signal for maintaining a data transmission rate in a TDD or TDMA system and a communication system adopting the same.
A transmitter of a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system is required to satisfy Out Of Band Emission (OOBE) characteristics such as an Adjacent Channel Leakage Ratio (ACLR). Even though items used and levels required in individual communication standards may be different, almost communication standards define OOBE characteristics that a transmitter should satisfy. This is for preventing other communication links from being damaged due to an operation of a specific transmitter.
In order for a transmitter to satisfy OOBE characteristics defined by a communication standard, firstly, a digital input signal to a Digital to Analog Converter (DAC) should satisfy the defined OOBE characteristics. This means that the resulting signal, when the ideal up-conversion for a digital input signal to a DAC is assumed, satisfies the OOBE characteristics. Next, degradation of OOBE characteristics after passing through analog and RF stages should be within an acceptable range, so an output signal of the transmitter finally should satisfy OOBE characteristics defined in the communication standard.
Using a filter is the most general signal processing method for a digital signal to satisfy the OOBE characteristics defined in communication standards. Such a filter attenuates a signal out of a transmission channel and is called as a channel filter. For an Orthogonal Frequency Division Multiplexing (OFDM) signal, the same effect as the use of channel filter may be obtained by using a window. However, windowing is not available for other signals except for the OFDM signal. A channel filter is a Finite Impulse Response (FIR) filter in most cases. In addition, since a digital filter is generally implemented through discrete convolution between a unit pulse response of the filter and input samples to the filter, the number of samples in a time domain increases by amount of an order of the filter after FIR filtering.
An increase of the number of samples in a time domain by amount of the order of channel filter after channel-filtering does not matter in case that a multiple access scheme is not a Time Division Multiple Access (TDMA) in a Frequency Division Duplex (FDD) system. However, in case of a Time Division Duplex (TDD) system or a multiple access scheme of TDMA, an increase of the number of samples in a time domain after channel-filtering decreases a data transmission rate in principle. In other words, a data transmission rate decreases because a part of time slot during which a signal is transmitted in a TDD or TDMA system is allocated to samples added by a channel filter used for satisfying OOBE characteristics, not to information to be delivered.